have had a/c charge checked and a/c comp.turns off & on( 50-55deg @vent)in truck never gets cold pls help

Sounds like the system is either undercharged or overcharged. Put a good set of R134a gauges on it. Your outlet vents should be down to about 40 degrees with an outside temp of 80. (Hint: if the system is a bit undercharged, the evaporator could sometimes ice up, which would block the flow of air.)

Problem seem to be worse in slow, stop and go traffic? .. How does it do out on the open highway while running @ a 'fairly constant' speed (≈) 55 mph?